This invention relates to a semiconductor driver circuit formed of MOS transistors and other elements.
Conventionally known is a driver circuit having a bootstrap circuit as shown in FIG. 1, for example. The driver circuit of FIG. 1 includes switching MOS transistors 2 and 4 whose gates are both connected with an input terminal V.sub.IN and whose sources are grounded; an enhancement-type MOS transistor 6 and a depletion-type MOS transistor 8 whose current paths are connected in series between a power supply terminal V.sub.D and with the drain of the MOS transistor 2 and a D-type MOS transistor 10 connected between the power supply terminal V.sub.D and the drain of the MOS transistor 4. The gate of the MOS transistor 6 is connected to the power supply terminal V.sub.D, and the gates of the MOS transistors 8 and 10 are both connected with the drain of the MOS transistor 2. The driver circuit further includes a capacitor 12 connected in parallel with the current path of the MOS transistor 4, and a capacitor 14 connected between the gate and source of the MOS transistor 10. An output terminal V.sub.OUT of the driver circuit is coupled with the drain of the MOS transistor 4.
Assume now that a high-level voltage is applied to the input terminal V.sub.IN. In this case, the MOS transistors 2 and 4 are turned on to allow current to flow through the MOS transistors 6, 8 and 2 as well as through the MOS transistors 10 and 4, and capacitors 12 and 14 are kept in a discharged state. Subsequently, when a low-level voltage is applied to the input terminal V.sub.IN, the MOS transistors 2 and 4 are turned off, and the capacitors 12 and 14 are charged through the MOS transistors 6 and 8. In this case, if the capacitor 12 has a capacitance greater enough than that of the capacitor 14, the voltage across the capacitor 14 becomes (V.sub.D -V.sub.T) (here V.sub.T is the threshold voltage of the MOS transistor 6). As the charging voltage of the capacitor 14, i.e. the gate voltage of the MOS transistor 10 increases, the resistance of the MOS transistor 10 is reduced, so that the capacitor 12 is charged more quickly through the MOS transistor 10.
The charging voltage of the capacitor 14 or the gate voltage of the MOS transistor 10 is further boosted by a bootstrap action of the capacitor 12. As a result, the resistance of the MOS transistor 10 is further reduced, and finally the capacitor 12 is charged up to the supply voltage V.sub.D, and the gate voltage of the MOS transistor 10 becomes (2 V.sub.D -V.sub.T).
In an actual driver circuit, however, leakage current flows through the MOS transistors 2 and 4 in the nonconductive state, so that the gate potential of the MOS transistor 10 can be increased to only about (3/2 V.sub.D -V.sub.T) at the most. Accordingly, the conductive resistance of the MOS transistor 10 cannot be made small enough. When the high-level input voltage is applied to the input terminal V.sub.IN, moreover, relatively large power consumption may be caused by current flowing through the MOS transistor 10.